Unable to pair iPhone 15 to iPad mini 4

You can't pair an iPhone to an iPad. What exactly are you hoping to accomplish? You can Hotspot from an iPhone to an iPad if you need to provide Wifi to your iPad. But in the traditional sense of Bluetooth pairing that isn't an option. As long as the iPhone and iPad are on the same Apple ID, there is no reason to need to pair them.

Unable to pair iPhone 15 to iPad mini 4.

This is correct. You cannot "pair" an iPhone to an iPad using Bluetooth, if that is what you are trying to do.